1. Field of the Invention
The present invention relates generally to a miniature accelerometer that can be used in aircraft, helicopter or automobile navigation, for example, or for improved power assistance of braking or active suspension of terrestrial vehicles.
The invention relates more particularly to a monolithic accelerometer including a fixed part, two mobile mass parts referred to as test weights, and two resonators, each of which has one end fastened to one of the two mobile mass parts.
2. Description of the Prior Art
The resonators constituting the responsive members of the accelerometer according to the invention are preferably flexional or torsional vibratory blades of piezoelectric material. The vibration frequencies of each of the blades are highly sensitive to the tensile or compression force that is exerted longitudinally on the blade when the inertial mass fastened to it subjected to acceleration. The extension of the blades and the compression of the other blade are converted into electrical signals that are picked up by electrodes supported by the vibratory blades and connected to two oscillator circuits, for example. A signal at a differential frequency whose variations are representative of those of the acceleration is produced at the output of the oscillator circuits. The benefit of using the difference between the two frequencies is that this reduces the effect on the two blades of spurious common mode inputs, for example temperature.
Another important aspect is the monolithic nature which enables miniature accelerometers to be fabricated at relatively low cost by chemical machining and promotes good performance, since the process of assembling together component parts generally constitutes a major limitation of non-monolithic accelerometers. The materials most frequently used to make monolithic accelerometers are quartz and silicon, which are appreciated for the excellent stability of their mechanical characteristics.
FIG. 1 shows an accelerometer of the above type disclosed in U.S. Pat. No. 4,945,765. The body of this accelerometer 14 is monolithic and is obtained by chemically machining a silicon plate. The body includes a fixed part 18, two inertial masses 20 and 22, two resonators 28 and 30 and two hinges 24 and 26. The resonators 28 and 30 vibrate in torsion and are excited electrostatically by means of a device (not shown) at whose output their resonant frequencies are delivered. The direction of sensitivity of the accelerometer is close to perpendicular to the faces of the plate. Acceleration applied in this direction causes a tension force to one resonator and a compression force to the other resonator, and the output signal of the accelerometer is the difference between the frequencies of the two resonators. The mechanical design of the accelerometer 14 nevertheless has a drawback associated with the vibration of the two resonators 28 and 30. The alternating mechanical forces generated by the vibrations of the two resonators where they are xe2x80x9cbuilt intoxe2x80x9d the fixed part 18 lead to dissipation of vibratory mechanical energy in the fixed part. This reduces the Q quality factor of the vibration of each of the resonators 28 and 30. This affects the precision of the measurement of the differential frequency and therefore the value of the acceleration deduced therefrom.
FIG. 2 shows another accelerometer disclosed in our U.S. Pat. No. 5,962,786. The body of the accelerometer ADxe2x80x2 is monolithic and is obtained by chemically machining a quartz plate. This body includes a fixed part 1xe2x80x2 with an I-shaped face contour, four U-shaped mobile mass parts comprising two inertial masses 21 and 22 and two resonators 31 and 32, four parallelepiped-shaped articulation blades 811, 821, 812, 822 and two flexible frames 51 and 52. The resonators 31 and 32 vibrate in flexion and are excited piezoelectrically by means of a device (not shown) at whose output their resonant frequencies are delivered. The direction of sensitivity of this accelerometer is close to perpendicular to the faces of the plate. Acceleration applied in this direction causes a tension force to one resonator and a compression force to the other resonator, the output signal of the accelerometer being the difference between the frequencies of the two resonators. This accelerometer does not have the drawback of dissipation of vibratory mechanical energy in the fixed part because the flexibility of the frames 51 and 52 provides a mechanical filtering effect between the resonators and the fixed part. Also, the accelerometer eliminates coupling between the two resonators (see U.S. Pat. No. 5,962,786, col. 4, lines 13-15). This accelerometer is therefore very suitable for industrial applications that require excellent precision and moderate cost. On the other hand, it has drawbacks in applications which require very low fabrication costs, in particular the field of automotive engineering. The relative complexity of the structure shown in FIG. 2 impacts on the yield of fabrication by chemical machining and limits the possibilities of miniaturization, which limits the number of structures that can be made on a quartz wafer of given dimensions. These drawbacks make it impossible to obtain a very low fabrication cost.
The present invention proposes a geometrical shape which prevents leakage of vibratory mechanical energy from the resonators to the fixed part and is more suitable for miniaturization. This reduces the fabrication cost and satisfies industrial requirements for very cheap accelerometers offering high performance.
According to the invention, this monolithic miniature accelerometer comprising a fixed part, two first mobile mass parts referred to as inertial masses, two hinge blades each having one end fastened to one of the two mobile mass parts, and two resonators each having one end fastened to one of the two mobile mass parts, is characterized in that it comprises a third mobile mass part fastened to the other end of each of the two resonators and of each of the two hinge blades, and a flexible stem situated between the first two mobile mass parts and connecting the third mobile mass part to the fixed part.
Locating the stem between the two inertial masses helps to maximize the total mass of the mobile parts. The flexibility of the stem combined with the total mass of the three mobile parts provides a mechanical filter between the resonators and the fixed part of the accelerometer. The Q quality factors of the resonators is therefore not degraded much and the precision of the measurement is excellent. The simplicity and compactness of the structure achieved by locating the stem between the first two mobile mass parts is also beneficial for miniaturization and achieving a good fabrication yield. On the other hand, the presence of a mobile mass part common to the two resonators rules out eliminating mechanical coupling between them and a different solution must be found to resolve this problem thereby maintaining the precision of the accelerometer.
According to a preferred embodiment, the flexible stem is a beam extending substantially parallel to the resonators and whose height is significantly greater than the dimensions of its cross section.
To maximize the efficiency of the mechanical filter, the longitudinal axis of symmetry of the flexible stem is substantially an axis of symmetry of the body of the accelerometer.
To exploit its performance optimally, the accelerometer is preferably fixed to a base whose larger faces are not parallel, which enables the axis of sensitivity of the accelerometer to be strictly perpendicular to the plane of the support.